Janczarek Monika, Skorupska Anna
Department of Genetics and Microbiology, Institute of Microbiology and Biotechnology, University of Maria Curie-Skłodowska, Akademicka 19 st., Lublin, 20-033, Poland; E-Mail:
Int J Mol Sci. 2011;12(6):4132-55. doi: 10.3390/ijms12064132. Epub 2011 Jun 22.
The acidic exopolysaccharide (EPS) secreted in large amounts by the symbiotic nitrogen-fixing bacterium Rhizobium leguminosarum bv. trifolii is required for the establishment of an effective symbiosis with the host plant Trifolium spp. EPS biosynthesis in rhizobia is a very complex process regulated at both transcriptional and post-transcriptional levels and influenced by various nutritional and environmental conditions. The R. leguminosarum bv. trifolii rosR gene encodes a transcriptional regulator with a C(2)H(2) type zinc-finger motif involved in positive regulation of EPS synthesis. In silico sequence analysis of the 450-bp long rosR upstream region revealed the presence of several inverted repeats (IR1 to IR6) and motifs with significant identity to consensus sequences recognized by PhoB and LysR-type proteins associated with phosphate- and flavonoid-dependent gene regulation in R. leguminosarum. Using a set of sequentially truncated rosR-lacZ transcriptional fusions, the role of the individual motifs and the effect of phosphate and clover root exudates on rosR expression were established. In addition, the significance of IR4 inverted repeats in the repression, and P2-10 hexamer in the activation of rosR transcription, respectively, was found. The expression of rosR increased in the presence of phosphate (0.1-20 mM) and clover root exudates (10 μM). PHO boxes and the LysR motif located upstream of the rosR translation start site were engaged in the regulation of rosR transcription. The synthesis of EPS and biofilm formation decreased at high phosphate concentrations, but increased in the presence of clover root exudates, indicating a complex regulation of these processes.
共生固氮细菌三叶草根瘤菌大量分泌的酸性胞外多糖（EPS）是与宿主植物三叶草属建立有效共生关系所必需的。根瘤菌中EPS的生物合成是一个非常复杂的过程，在转录和转录后水平均受到调控，并受各种营养和环境条件的影响。三叶草根瘤菌的rosR基因编码一种具有C(2)H(2)型锌指基序的转录调节因子，参与EPS合成的正调控。对rosR上游450 bp长区域的电子序列分析揭示了几个反向重复序列（IR1至IR6）以及与三叶草根瘤菌中与磷酸盐和类黄酮依赖性基因调控相关的PhoB和LysR型蛋白识别的共有序列具有显著同源性的基序。使用一组连续截短的rosR-lacZ转录融合体，确定了各个基序的作用以及磷酸盐和三叶草根系分泌物对rosR表达的影响。此外，还分别发现了IR4反向重复序列在rosR转录抑制中的意义以及P2-10六聚体在rosR转录激活中的意义。在存在磷酸盐（0.1 - 20 mM）和三叶草根系分泌物（10 μM）的情况下，rosR的表达增加。位于rosR翻译起始位点上游的PHO框和LysR基序参与了rosR转录的调控。在高磷酸盐浓度下，EPS的合成和生物膜的形成减少，但在存在三叶草根系分泌物的情况下增加，这表明这些过程受到复杂的调控。
